Today is the winter solstice — specifically, it occurs at 23:38 GMT, or 6:38 p.m. Eastern (US) time. Technically what this means is that, at that moment, the center of the Sun is at the lowest declination of the year.

Um, what?

Astronomers measure the positions of objects on the sky using a coordinate system based on longitude and latitude on Earth, but to make it more confusing we call it Right Ascension and declination. The first measures the east/west position, and the second north/south. So just like an island might have coordinates of so-and-so degrees longitude and this-and-that degrees latitude, a star has an RA and a dec (if you want to sound cool and use astronomy slang).

OK. But what if an object moves? Well then, the coordinates change with time, too (just like the lat and long of a ship steaming across the ocean has changing coordinates). As the Earth orbits the Sun, the Sun’s position against the background stars changes, and therefore the coordinates change. I won’t go into details — you can read about it here, for example — but the Sun makes a sine wave across the sky over the course of the year, sometimes farther north, sometimes south. The time when the Sun’s position (actually, the center of the Sun’s disk) is at its farthest south — -23° declination — is called the winter solstice.

And that’s where we are today. The declination of the Sun has been getting lower ever since last June, and today it reaches its farthest point south. At 11:38 p.m. GMT, the Sun’s movement south stops (solstice literally means "Sun stops/stands still"), and it starts to slowly creep back north again. That’s why there is an actual moment, a point in time, for the solstice. In June it’ll get as far north as it can, and the process reverses. Incidentally, the times halfway between the solstices when the Sun’s declination is exactly 0 are called the equinoctes (the singular is equinox). Just so’s you know.

I’ll note the eclipse last night had nothing to do with the solstice at all, despite the hype. It’s just another day of the year as far as the Moon is concerned, so we get eclipses on the solstices every now and again, just like any other specific day. Heck, for me in Boulder, the first half hour of the eclipse was on the day before the solstice, and people on the west coast had the eclipse going for an hour and a half before midnight changed the date! That just shows you that we sometimes get excited about arbitrarily forcing our units (days starting at midnight) on the Universe, when honestly it couldn’t care less.

That doesn’t make the solstice any less cool, though. Every day now for people in the northern hemisphere the Sun will stay up a little bit longer, and days will stretch out, and things will warm up once again. It’s a time of renewal, which is no doubt why so many ancient cultures celebrated it.

And for you folks in the southern hemisphere? Well, take the inverse of what I said: the Sun is at its highest point in your sky now, the days will get shorter, and it’ll get colder. But never fear! Things will turn around once again, as they always have, and they always will.

Actually it’s not true that the days get warmer for you or colder for us from now. The daylight length changes, but there’s a lot of thermal lag in the earth, as well as local climate effects. For me, January and February are usually much hotter than December. And Tim, yes.

Today is the winter solstice … And for you folks in the southern hemisphere? Well, take the inverse of what I said: the Sun is at its highest point in your sky now, the days will get shorter, and it’ll get colder.

Personally, I always notice the day of earliest sunset which is already a couple of weeks ago (usually early December…hard to pin down the exact date since most sunset tables only show times to the nearest minute and there are several days where the minute doesn’t change…from November 28th to December 9th sunset is listed as 5:18pm in Tucson). Now sunset is 5:23pm but we keep losing daylight because sunrise is getting later faster than sunset!

Well, up until the point where the Sun becomes a red giant, and either the Earth will be incinerated in the expanded sphere of the Sun’s influence, or it gets tossed into interstellar space.

My understanding of this – and, of course, I could be wrong – is that while the Earth may be consumed by the red giant sun phase (if we fail to shift its orbit) it won’t necessarily be ejected from the solar sytem if it survives but rather will exist as a burnt out cinder in a larger orbit.

There are a few complicating factors that come into play here such as :

1.) Our Sun losing mass in the process of ballooning outwards and that resultant reduced (and ever-reducing) mass causing the orbit of the planets incl. ours to expand outwards away from its expanding and reddening and cooling yellow sub-giant to ornage giant to full “second ascent” red giant and finally Mira variable surface.

2.) Tidal effects notably the likelihood that the Earth will raise a tidal bulge on the red giant solar surface and this will cause our globe to spiral in to destruction

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3.) Possible orbital chaos and interactions between the inner planets – there is a very remote chance that Mercury’s orbital eccentricity will cause a chain reaction where it destabilises the oribts of the inner four rock dwarf planets.

So, the upshot of all this is that while Mercury and Venus definitely seem doomed and Mars will almost certainly be safe – albeit overcooked – there’s considerable uncertainty about Earth’s ultimate fate.

The likelihood seems to be about 50/50% (personal very rough guesstimate) that our world will be either consumed or left orbiting our Sun’s stellar corpse – a white dwarf which may have around half or less its present mass.

Until, given a long enough time span, the scorched remains of our solar system get further disrupted by encounters with passing stars, black holes or brown dwarfs or the galctic merger with M31 (Andromeda galaxy) or, just possibly, eventually after trillions upon trillions of years the dissolution of the protons composing matter itself.

Or who knows?

Several interesting studies have been done on this fascinating question arriving at a number of possible answers.

“…but the Sun makes a sine wave across the sky over the course of the year…”

Actually, if you were patient enough to record the position of the sun at exactly noon (12:00:00pm) every day for 365 days (okay 365.25 days) you would come up with a figure 8 “double loop” plot not unlike a figure skater would create on the ice, for example.

Depending on your hemisphere, the point where the sun is located on this “double loop” would be at the apex or most extreme north or south point on your plot. Where the two loops meet in the middle are the equinoctes (thanks for the plural form).

The sine wave metaphor mostly applies for the moon as it orbits around the earth and both orbits the sun. If one plots the position of the moon around the earth for the whole year, he/she would come up with a sinusoidal pattern and not a figure eight.

which pessimistically suggests we may only have a half-billion years of habitabity left.

Of course, as a (semi?)intelligent species we’re perhaps beginning to have a serious say in our own planet’s astronomical and biological fate given the both the potential good we can do – eg. preventing comet and asteroid impacts – and potential harm we can do, eg. nuclear wars.

“Every day now for people in the northern hemisphere the Sun will stay up a little bit longer, and days will stretch out, and things will warm up once again. It’s a time of renewal, which is no doubt why so many ancient cultures celebrated it.”

Which is why I believe today should be celebrated as the start of Spring, as it’s the start of days getting longer. Heck, I would even campaign for today to be the start of the new year while we’re at it.

Combining my love of meteorology and astronomy, I much prefer the meteorological seasons. Winter is December 1 to February 28/29. They are more accurate in terms of weather. I’ve always looked at December 21st being the start of winter as confusing. Wouldn’t it make more sense if this was the middle of winter today?

& (#15) 24601 :

Which is why I believe today should be celebrated as the start of Spring, as it’s the start of days getting longer. Heck, I would even campaign for today to be the start of the new year while we’re at it.

Personally I’m happy using the three monthly divisions :

Summer = December, January, February

Autumn = March, April, May

Winter = June, July, August

Spring = September, October, November

Which works fine for me being convenient and fairly well matching the usual weather conditions for 35 degrees South latitude. (Adelaide, South Australia.)

Of course, “seasonality” (if that’s word) really depends on your individual geographic location & climatic zone – fer ex further north in Oz there’s just the two seasons, The Wet & The Dry, which also applie sinasense to polar regions 6 months lightness / darkness whilst some tropical zones are the same year round experiencing no real seasons at all as I understand it.

By way of comparison :

“During its summer, the frozen nitrogen on Pluto evapourates to
create a temporary atmosphere. With the onset of winter the nitrogen
turns to frost and falls back to the surface. On Pluto the winter
weather doesn’t merely deteriorate – it completely disappears.”
– Page 19, ‘The Planets’, McNab & Younger, BBC Worldwide Ltd., 1999.

Which might put some cold weather some folks are curently experiencing in some perspective! 😉

Man, I wish I was in Antarctica. Midnight sun! That must have been so cool…

…Pun unintended.

Messier Tidy Upper @ 18, definitely all for that! (And it – mostly – works for Sydney, too.) Certainly, the weather doesn’t reflect northern spring starting today. I’m certainly not about to go to Montreal in a few weeks and wear springtime clothing, I’d freeze to death! (January tends to average around -10 C. Brr!)

At the winter solstice, the rising sun shines all the way into the middle of Newgrange, a 5000 year old neolithic passage tomb mound located north of Dublin, Ireland. That is, if it’s clear, which is not the case most years, including this one.

I’ll note the eclipse last night had nothing to do with the solstice at all, despite the hype.

Maybe I’m getting my hype from the wrong place, but all of my news sources said that the eclipse falling on the solstice was an interesting coincidence, and nothing more. Was someone saying it was significant?

BA said: “The time when the Sun’s position (actually, the center of the Sun’s disk) is at its farthest south — -23° declination — is called the winter solstice.”

If the center of the Sun’s disk is at its southernmost, isn’t the “top” and “bottom” of the Sun at their respective southernmost points as well? I mean, is there some reason to be specific about the center thing instead of just saying the Sun is at its farthest south?

Actually, if you were patient enough to record the position of the sun at exactly noon (12:00:00pm) every day for 365 days (okay 365.25 days) you would come up with a figure 8 “double loop” plot not unlike a figure skater would create on the ice, for example.

@min84 – I’m guessing that the measurement is taken at the center of the sun due to atmospheric distortion in the sun’s appearance at the extremes. (Am I right? Or was this a “nice try, but leave astronomy to the professionals?”)

And I had a blast watching the eclipse last night. The rain and clouds cleared enough for me to see it… a little foggy at times, but spectacular.

I really wish I was joking here – but I’m not…
A couple of days ago, the news page of my supposedly “reputable” ISP, in the UK, carried yet another update on our freezing weather, which included the following statement ( emphasis mine ):

“It may get even colder on Tuesday, with the winter solstice, the time at which the Earth is furthest from the Sun.”

Er – WHAT???????????????????????????!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I kid you not, folks; this is an example of the state of today’s education system! The imbecile who wrote this apparently thinks that summer and winter are caused by the Earth being closer to and further from the Sun!!!!!!!!!!!!!!!! I guess he isn’t even aware of the simple fact that the seasons are reversed in the southern hemisphere, and it only is the winter solstice in the northern!
I despair. Is it any wonder that so many people labour under idiotic misconceptions, when the media propagate this sort of drivel?

( Just for the record, in case anyone doesn’t know; the Earth is in fact closest to the Sun at the beginning of January, and furthest from it at the beginning of July. An amazing number of people in the northern hemisphere seem to find that surprising, for the reasons mentioned above. Of course, while the Earth’s orbit is an ellipse, it’s actually so close to a circle that if it was drawn to scale, you would have to look closely to tell the difference; the difference in temperature due to the variation in distance is miniscule, compared to that due to the real cause of the seasons. )

When I was a kid, we learned about the axial tilt and the cause of the seasons at the age of 12. ( I personally understood it long before that, due to my interest in astronomy. ) FSM knows what they teach kids at school these days!

Man, I wish I was in Antarctica. Midnight sun! That must have been so cool…

A colleague of mine is right now at the south pole for a “Winter over”, which means that he stays there for 13 (!!!) months (and he just made 1 so far…).

However, I am glad that we finally reversed the motion of the sun 😉 . I prefer the warmth of summer. Although a white Christmas is also a beautiful thing, and we are going to have the first white Christmas where I live since a very long time (I cannot remember the last time).
Normally, German winter su**s, with +3°C and rain…… In the last couple of years this has changed a bit, and this is also a very cold winter so far (not counting that it officially begins only today…). I enjoy it, actually, although most people are freaking out due to the snow and have no idea what to do….. Crazy folks….

If the center of the Sun’s disk is at its southernmost, isn’t the “top” and “bottom” of the Sun at their respective southernmost points as well? I mean, is there some reason to be specific about the center thing instead of just saying the Sun is at its farthest south?

Yes, because he also gave a specific declination of -23 degrees, and that number refers to the center of the sun’s disk.

But the December solstice marks the point at which the evenings start to get longer faster than the mornings are getting shorter (that is, each day is a bit longer even though both endpoints are still moving later in the day). Sunrise will soon start to get earlier each day.

This is all to do with the equation of time (the difference between apparent solar time – where the sun is in the sky – and mean solar time – where it is on average over the year). Or to put it another way, the time when the sun is at its highest point in the sky each day (local noon) can be earlier or later than the time at which the clock shows 12:00. See http://en.wikipedia.org/wiki/Equation_of_time At the moment, the sun is lagging behind the clock, but it is catching up quickly, so sunrise and sunset are both getting later each day. The sun will soon be ahead of the clock, and the rate of change will get slower, and the changing daylength will dominate over the equation of time.

I’ll note the eclipse last night had nothing to do with the solstice at all, despite the hype.

It’s worse than that. Around here the hype was about the “rare triple play” of the solstice, full moon, and eclipse.

Okay, I suppose you could say it’s more than just the “double play” of solstice and full moon. But, a lunar eclipse is always at the time of the full moon, just as a solar eclipse is always at the time of a new moon.

Technically what this means is that, at that moment, the center of the Sun is at the lowest declination of the year.

Given that the Earth’s orbit is not a circle, and therefore the Sun’s apparent size changes as well (hence the use of its center as the reference point), when does the “bottom edge” of the Sun reach its lowest declination?

I’m still looking for a simple explanation of why the time of sunsets started to get later last Tuesday, a week before the “shortest day of the year.” The explanations I have seen don’t give me a good picture of the geometry in my head.

One thing advocates of climactic seasons (vs. astronomical seasons) should be aware of is that climactic seasons are not the same everywhere (or exactly opposite in the other hemisphere.) For example, climactic winter most definitely does not end in New England at the end of February. Most or all of March (including well after the equinox) is much more like winter than spring. (Except in 2005, when the crocuses came up in mid-February.)

Spring is April, May and part of June (and half of that is really Mud season), Summer is mid-June through mid-September, and Fall is mid-September through late November. Winter is 4+ months, Summer is 3 months, and Spring and Fall (the nice times) are 2 1/2 each, if we get lucky.

For other parts of North America, it is quite different, let alone other continents. Some areas don’t have climactic seasons at all, others have Dry and Monsoon.